There is a great Yii2 widget called multi-level-horizontal-menu by Marc Oliveras, When you want to change CSS, simply modify the css file may not work right away. Here is the step works for me. In my project root, I have folder (1) host33->multilevelhorizontalmenu->resources->style.css, (2) web->assets->(4ff3d74f, 5f5aecf0,d5cb6f, f8623b5f), step 1) delete all four or whatever number of folders under web->assets, step2) make change on the file host33->multilevelhorizontalmenu->resources->style.css, step3) refresh your browser, it’s done, you will see the change on your page.
Everytime you refresh your browser, these folders 4ff3d74f, 5f5aecf0,d5cb6f, f8623b5f,… will automatically be generated by Yii2, so if you want to make another change on the css file, you need to repeat step1, 2,3.
It’s a bit hassle but since once you have the menu setup right , you normally don’t change it that often so it still worth it.